Balsa core laminate having bevelled edges

ABSTRACT

A structural laminate in which an end-grain balsa core panel is sandwiched between and adhered to facing skins that cover the edges of the panel as well as its opposing faces. To make it possible for facing skins that cannot be sharply bent to conform to edges of the panel, these edges are bevelled to impart a gentle slope thereto. Bevelling is effected by means of a high-pressure die whose angled face has a staircase formation defined by a series of right-angle microsteps. When the die is forced against a right angle edge of the core panel to be bevelled with a pressure exceeding the compressive strength of the balsa, each microstep then imposes pressure in a direction parallel to the axes of the cluster of internal balsa cells engaged by the microstep to more or less collapse this cluster to an extent depending on the staircase position of the microstep, thereby densifying the internal structure of the balsa at the edge of the panel without disintegrating this structure.

BACKGROUND OF INVENTION

1. Field of Invention

This invention relates generally to a structural laminate in which anend-grain balsa core panel is sandwiched between facing skins, and moreparticularly to a laminate of this type in which edges of three corepanels are bevelled and in which the facing skins fully cover theseedges as well as the opposing faces of the core panel.

2. Status of Prior Art

Balsa has outstanding properties unique in the field of lumber, for onthe average it weighs less than nine pounds per cubic foot, this being40% less than the lightest North American species. Its cell structureaffords a combination of high rigidity and compressive and tensilestrength that is superior to any composite or synthetic material ofequal or higher density. Balsa is dimensionally stable and may beprocessed by standard woodworking techniques.

It is known that end-grain balsa wood, because of its high compressivestrength, is capable of supporting far greater loads than flat-grainmaterial of the same density, and that low-density balsa in theend-grain direction will support greater loads than flat-grainedmaterial of higher density. The cellular structure of balsa is such thatthe number of cells per cubic foot is extremely high, the wall thicknessof each cell being quite thin. The cells are effectively independent ofeach other, each cell being comparable to an independent column orfiber. The cells of balsa wood are substantially parallel to each other.

Structural sandwich laminates can be created by bonding thin facings orskins to balsa wood panels which function as a core. Thus the Kohn etal. U.S. Pat. No. 3,325,037 and the Lippay U.S. Pat. No. 3,298,892disclose structural sandwich laminates whose core is formed of end-grainbalsa. The resultant laminates having a remarkably highstrength-to-weight ratio as well as excellent thermal and acousticinsulation properties.

Of particular prior art interest is the Kohn U.S. Pat. No. 4,343,846which discloses a structural laminate in which an end-grain panel islaminated to composite facing sheets formed by synthetic plastic filmsreinforced by glass fibers.

End-grain balsa-core sandwich laminates are widely used intransportation and handling equipment, such as for floors of railroadcars, shipping containers, cargo pallets, bulkheads, doors and reeferbodies, as well as in a variety of other applications. These laminatesare also employed for structural insulation in aircraft applications, inhousing and in boating.

While it is known to use thin metal facings or plastic skins instructural laminates having an end-grain balsa core, in recent years ithas become the practice to enhance the strength of the laminate by meansof composite facing skins reinforced by carbon or graphite fibers. Alsoin use are facing skins made of Kelvar, a synthetic plastic ofexceptionally high strength.

For many installations, such as in automotive flooring applications, itis essential that the facing skins laminated to the end grain balsa corenot only cover opposing faces of the panel, but also its edges so thatthese edges are protectively shielded and not exposed.

Composite facing skins which incorporate relatively stiff reinforcingfibers, though flexible, cannot be sharply bent. Should, therefore, oneseek to bend a composite facing skin placed on a face of a core panel soas to conform to a right-angle edge of this panel, the skin cannot befully bent. As a consequence, an air pocket will be created between theedge and the bent skin which introduces an unacceptably weak point inthe structural laminate.

It therefore becomes desireable to bevel the edges of the end-grainbalsa core panel to impart a gentle slope thereto. This slope makes itpossible to laminate a facing skin which cannot be sharply bent yet canbe made to fully conform to the gentle slope of the core panel and leaveno air pocket.

Though one may bevel the edges of the panel by means of a router machinehaving a rapidly revolving spindle and a cutter, the use of this machinefor this purpose has serious drawbacks. A milling operation of this sortis difficult to perform and is time consuming, thereby addingsubstantially to the cost of producing the structural laminate.Moreover, milling of the end grain balsa core edges disintegrates thewood and yields balsa waste, giving rise to pollution problems in theproduction plant.

SUMMARY OF INVENTION

In view of the foregoing, the main object of this invention is toprovide a structural sandwich laminate in which an end-grain balsa corepanel having bevelled edges is sandwiched between facing skins thatcover these edges as well as the opposing facing of the panel.

More particularly, an object of this invention is to provide a techniquefor bevelling the edges of the end-grain balsa core panel so as toimpart a gentle slope thereto which makes it possible for compositefacing skins which cannot be sharply bent to fully conform to the edgesof the core panel and leave no air pockets.

A significant advantage of a structured laminate in accordance with theinvention is that it can be produced at relatively low cost, forbevelling of the edges is quickly effected by a die-stamping operationthat imparts a gentle slope thereto which densifies the edges withoutdisintegrating the structure of the balsa and producing balsa waste.

Briefly stated, these objects are attained by a structural laminate inwhich an end-grain balsa core panel is sandwiched between and adhered tofacing skins that cover the edges of the panel as well as its opposingfaces. To make it possible for facing skins that cannot be sharply bentto conform to edges of the panel, these edges are bevelled to impart agentle slope thereto. Bevelling is effected by means of a high-pressuredie whose angled face has a staircase formation defined by a series ofright-angle microsteps. When the die is forced against a right angleedge of the core panel to be bevelled with a pressure exceeding thecompressive strength of the balsa, each microstep then imposes pressurein a direction parallel to the axes of the cluster of internal balsacells engaged by the microstep to more or less collapse this cluster toan extent depending on the staircase position of the microstep, therebydensifying the internal structure of the balsa at the edge of the panelwithout disintegrating this structure.

BRIEF DESCRIPTION OF DRAWINGS

For a better understanding of the invention as well as other objects andfurther features thereof, reference is made to the following detaileddescription to be read in conjunction with the accompanying drawings,wherein:

FIG. 1 is a perspective view of a conventional end-grain balsa corepanel having right-angle edges;

FIG. 2 is a magnified view of the vertical cells forming the internalstructure of the end grain balsa;

FIG. 3 is a sectional view of composite skins laminated to aconventional core panel having right angle edges to form a structurallaminate;

FIG. 4 is a section taken through a structural laminate in accordancewith the invention in which an edge of the core panel is bevelled andthe facing skins conform thereto;

FIG. 5 schematically illustrates a high-pressure die having an angledface in accordance with the invention for imparting a gentle slope tothe right-angle edge of a conventional end grain balsa core panel; thedie being shown at a position directly above the edge;

FIG. 6 schematically shows the die pressed down against the right angleedge to impart a gently-sloped stepped bevel thereto;

FIG. 7 is a perspective view of the bevelled edge of the core panel;

FIG. 8 is a magnified view of the portion of the stepped bevel whichlies within circle 8C in FIG. 7, and

FIG. 9 illustrates a modified form of a stepped bevelled edge.

DETAILED DESCRIPTION OF INVENTION The Structural Laminate

In a structural laminate in accordance with the invention in which anend-grain balsa core panel is adhesively laminated between facing skins,the edges of the core panel are microstep-bevelled to impart a gentleslope thereto so that a facing skin which cannot be sharply bent can bemade to fully conform to these edges. By the term microstep-bevelled ismeant a bevel which does not have a planar surface, but a surface havinga staircase formation defined by a series of very small right anglesteps. Hence this bevel cannot be produced by a router or other millingmachine.

FIG. 1 illustrates a standard end-grain balsa core panel 10 having rightangle edges 11 and therefore unsuitable for use in producing astructural laminate in accordance with the invention. In order to renderthis panel usable for producing a laminate in accordance with theinvention, its edges must be microstep-bevelled in the manner to belater described.

Commercial balsa wood is normally kiln-dried to reduce its moisturecontent to about 10 to 12%. The steps necessary to kiln-dry wood and therecommended practices therefor are set out in Publication #188 of theU.S. Department of Agriculture Forest Service, Forest ProductsLaboratory. However, if kiln-dried wood is stored in a humidtemperature, its moisture content may rise substantially. In any case,even if the balsa wood to which skin is to be laminated has a moisturecontent of no more than 10%, because of hot curing necessary for apreferred epoxy adhesive, the heat will volatilize the moisture and theresultant vapors will interfere with effective lamination. Hence toeffect lamination with an adhesive that requires hot curing, prior tolamination to facing skins, the panel must first be kiln-dried to amoisture content of about 2 to 3%.

The cellular internal structure of end grain balsa, as shown in FIG. 2,is composed of vertical cells C each having a thin wall, the number ofcells per cubic foot being extremely high. The cells are effectivelyindependent of each other, each being comparable to an independentcolumn or fiber. The cells C are substantially parallel to each other.

If, as shown in FIG. 3, balsa core panel 10 having a right-angle edge 11is laminated to upper and lower composite facing skins 12 and 13 whichextend beyond right angle edge 11, and the upper skin 13 were bent downto cover edge 11, because of the strong stiff fibers included in theskin it could not be bent around this sharp edge. As a consequencethough skin 13 could then be adhesively laminated to the upper face ofcore panel 10, it could not conform to and be adhesively laminated toedge 11, for the skin would rupture at the bend 13B and there would becreated an entrapped air pocket 14 between the edge and the upper skin13. This region would constitute a weak point in the structurallaminate.

In a structural laminate in accordance with the invention, as shown inFIG. 4, each right angle edge of the end grain balsa core panel ismicrostep-bevelled to provide a gently-sloped edge 15, preferably aslope of about 45 degrees. Hence the upper composite skin 13 adhesivelylaminated to the upper face of core panel 10 may be bent to fullyconform and adhere to the sloped edge 15 of the core panel to provide ahigh-strength laminate free of air pockets and other points of weakness,the core panel being encased in the facing skins.

The portions of the interlaminated lower and upper facing skins 12 and13 which extend beyond the bevelled edge 15 may be severed in a verticalplane P adjacent the triangular apex of the bevelled edge or in a planespaced from this apex to form a laminate having a marginal flange.

Microstep Bevelling

As shown in FIG. 5, in order to impart a microstep bevel to eachright-angle edge 11 of a standard end-grain core panel 10 to render itsuitable for producing a structural laminate in accordance with theinvention, the panel is placed on the horizontal platen 16 of ahigh-pressure die press. The press is provided with ahydraulically-operated die 17 formed of stainless steel or otherhigh-strength material having an angled face 17F whose angle has thedesired gentle slope.

Face 17F of the die is machined to create a surface having a staircaseformation defined by a series of small right-angle microsteps S₁ toS_(n). When, as shown in FIG. 6, die 17 is forced down to engage theright-angle edge 11 of the balsa core panel and subjects thisright-angle edge to pressure that is higher than the compressivestrength of the end-grain balsa, then the right-angle edge of the balsais bevelled to conform to the shape of the die face. In practice, thedie pressure may be in excess of 1500 PSI.

Each right-angle microstep in the angled face 17F of die 17 imposes apressure only on that cluster of parallel balsa cells or columns whichare engaged by the microstep. This pressure is applied in a directionparallel to the parallel axes of the columnar cells in this cluster.

Thus, as shown in FIGS. 7 and 8, the pressure applied by each microstepin the die face acts to collapse and densify the cluster of cellsengaged thereby to an extent that depends on the position of themicrostep in the staircase. The highest step S₁ in the die staircaseonly slightly collapses the cluster of cells engaged thereby, while thelowermost step S_(n) effects a maximum collapse of the cluster of cellsengaged thereby.

Because the pressure applied by each die microstep is in a directionparallel to the parallel axes of the cluster of cells engaged theretoand these cells are effectively independent of each other, the collapseand densification of these cells does not disintegrate the internalstructure of the balsa. Hence no balsa waste results from this diestamping operation.

As shown in FIGS. 7 and 8, the bevelled edge 15 of the balsa core panelhas the desired gentle slope and its sloped surface has a staircaseformation corresponding to the staircase surface on the face of the die.

In practice, both the long and short right angle edges of a rectangularcore panel, or either the long edges or the short edges can be quicklybevelled in the manner described above to render the core panel suitablefor lamination to facing skins.

Modifications

It is not essential to the invention that the staircase formationimpressed on the right-angle edge of an end-grain core panel run in astraight line, as shown in FIGS. 5 to 8. Thus the die may have an angledface whose staircased surface runs along a curved line to produce whenthe die is pressed down against the right angle edge of the core panel,a cove-contoured bevelled edge 18, as shown in FIG. 9.

Bevelled edge 18 has a gentle slope that runs along a curved line,thereby avoiding the relatively sharp apex in the sloped edge 15 shownin FIG. 7 and making it possible to fully conform a facing skin to thiscove-contoured edge.

Advantages

One advantage of a microstep-bevelled edge in an end-grain balsa corepanel is that it acts effectively to roughen the surface of this edgeand therefore enhances the ability of a bonding agent such as an epoxyresin to adhere to this surface. Of greatest importance is the advantagegained by providing an angled die face in accordance with the inventionwhose surface is not smooth but is defined by a series of right anglemicrosteps, each of which in practice may have a narrow width of about1/16 th of an inch. Had the angled die face being formed by a smooth,planar surface, then when the die is subjected to high pressure andengaged the right-angle edge of the core panel to be bevelled, theresultant horizontal component of force would split the wood just as awedge acts to split firewood.

However, with a die whose angled face is defined by a serries of rightangle microsteps, when this die face is pressed to engage the edge ofthe end-grain balsa panel, the microsteps then exert forces parallel tothe end grain of the panel. Hence the microstepped die face generates nohorizontal force component that would cause the end grain panel tosplit.

As previously indicated, bevelling the edges of the end grain core panelmakes it possible to laminate facing skins thereto which cannot besharply bent, yet can be made to conform to the gentle slope of thebevelled edge. This gentle slope gives rise to another advantage, forthe resultant grandual transition from a cored laminate whose edge has agentle slope to an uncored solid laminate beyond the sloped edge reducesstress risers. These stress risers would occur had there been an abrupttransition from a core to an uncored laminate, as with a core panelhaving a right angle edge.

While there have been shown preferred embodiments of a balsa corelaminate having bevelled edges in accordance with the invention, it willbe appreciated that many changes may be made therein without departingfrom the spirit of the invention.

I claim:
 1. A structural laminate comprising:A. an end-grain balsa corepanel having an internal structure formed by substantially parallelcellular columns, said panel having an upper face at least one edge ofwhich is bevelled to impart a gentle slope thereto, the surface of saidedge having a staircase formation defined by a series of right anglemicrosteps; and B. a facing skin adhesively laminated to said upper faceof said panel and to said edge, said facing skin being incapable ofbeing sharply bent, but being bent to fully conform to said sloped edge.2. A laminate as set forth in claim 1, in which the facing skin is acomposite sheet that incorporates reinforcing fibers.
 3. A laminate asset forth in claim 2, in which the fibers are graphite fibers.
 4. Alaminate as set forth in claim 2, in which the fibers are glass fibers.5. A laminate as set forth in claim 1, in which said staircase formationruns along a straight line to define an edge having a triangular apex.6. A laminate as set forth in claim 1, in which the cellular columnsincluded in each of said microsteps are collapsed to an extent dependingon the position of the microstep in the staricase.
 7. A laminate as setforth in claim 1, in which said staircase formation runs along a curvedline to define a contoured edge.